After hydrocarbons have been removed from the ground, the fluid stream (e.g., crude oil, natural gas) is transported from place-to-place via pipelines. It is desirable to know with accuracy the amount of fluid flowing in the stream, and particular accuracy is demanded when the fluid is changing hands, or “custody transfer.” Ultrasonic flow meters may be used to measure the amount of fluid flowing in a pipeline, and ultrasonic flow meters have sufficient accuracy to be used in custody transfer.
The value of gas “changing hands” at the point of custody transfer in a high volume natural gas pipeline may amount to a million dollars or more in a single day. Thus, in some custody transfer situations a single meter body houses two independent ultrasonic flow meters. The two meters enable redundancy in case one meter fails, and in situations where both flow meters are operational, the accuracy of recorded flow volumes may be verified by comparing the two independent measurements. However, in some situations the meters may read differently even when each meter is operating correctly.